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Subjects

Abstract

Horizontal branch stars belong to an advanced stage in the evolution of the oldest stellar galactic population, occurring either as field halo stars or grouped in globular clusters. The discovery of multiple populations in clusters1,2 that were previously believed to have single populations gave rise to the currently accepted theory that the hottest horizontal branch members (the ‘blue hook’ stars, which had late helium-core flash ignition3, followed by deep mixing4,5) are the progeny of a helium-rich ‘second generation’ of stars6,7. It is not known why such a supposedly rare event8,9 (a late flash followed by mixing) is so common that the blue hook of ω Centauri contains approximately 30 per cent of the horizontal branch stars in the cluster10, or why the blue hook luminosity range in this massive cluster cannot be reproduced by models. Here we report that the presence of helium core masses up to about 0.04 solar masses larger than the core mass resulting from evolution is required to solve the luminosity range problem. We model this by taking into account the dispersion in rotation rates achieved by the progenitors, whose pre-main-sequence accretion disk suffered an early disruption in the dense environment of the cluster’s central regions, where second-generation stars form11. Rotation may also account for frequent late-flash–mixing events in massive globular clusters.

Acknowledgements

A.P.M. acknowledges support by the Australian Research Council through Discovery Early Career Researcher Award DE150101816. E.V. acknowledges support from grant NASANNX13AF45G. P.V. and F.D’A. acknowledge support from PRIN INAF 2011 “Multiple populations in globular clusters: their role in the Galaxy assembly” (principal investigator E. Carretta), and P.V. acknowledges support from PRIN MIUR 2010-2011, project “The Chemical and Dynamical Evolution of the Milky Way and Local Group Galaxies” (principal investigator F. Matteucci). T.D. acknowledges support from the UE Program (FP7/2007–2013) under grant agreement number 267251 of Astronomy Fellowships in Italy (ASTROFit). M.D.C. acknowledges support from INAF-OAR. A.B. acknowledges support from STScI grant AR-12656.

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Contributions

M.T., F.D’A., E.V. and M.D.C. jointly designed and coordinated this study. F.D’A. proposed and designed the rotational evolution model. E.V. designed and computed the dynamical simulation. M.T. and P.V. computed the new evolutionary models. A.D. computed synthetic colours with an internally consistent treatment of extinction across all bandpasses. M.T. and M.D.C. performed the simulations and the analysis. A.B. performed the data reduction and calibration for the WFC3/UVIS exposures. A.M. dealt with the optical data and the comparison with spectroscopic data. T.D. dealt with the problems connected to the modelling of stellar rotation. V.C. contributed to the discussion and to the writing of the text. A.D’E. and R.C.-D. provided insight on the dynamical aspects. All authors read, commented on and approved submission of this article.

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Editorial Summary

ω Centauri's blue hook explained

Some globular clusters, once thought to be composed of a single population of horizontal branch stars at an advanced stage of stellar evolution, also contain hot 'blue hook' stars. This study of the globular cluster ω Centauri demonstrates that the observed range of luminosities of the blue hook stars of ω Cen is successfully explained by a model in which their progenitors are second-generation helium-rich stars characterized by a range of rotation rates arising during the cluster's very early evolution.